Full Width Split Dropdown Button in Twitter Bootstrap

(example from the Twitter Bootstrap Docs). I now want to switch some of them to split buttons (example), but I can't seem to get the split buttons and the normal buttons to be the same length.

I have experimented with all kinds of things, but I can't seem to find a way to do this. Has anyone ever managed to do it? Keep in mind that I do not want to hard-code the width of any elements if I don't have to. (Note that this includes not using hard-coded percentages either.) If it is absolutely necessary, I am OK with defining a width for the toggle part of the button (because I know there will only be a single arrow character in there) but want to specify as few "magic numbers" as possible in order to keep the code maintainable in the future.